Hydrogen absorbing alloy electrode and process for fabricating same

ABSTRACT

A hydrogen absorbing alloy electrode characterized in that the electrode consists mainly of a mixture of a first hydrogen absorbing alloy powder A formed on surfaces of particles thereof with a surface layer  22  containing metallic copper or a copper oxide and a second hydrogen absorbing alloy powder B formed on surfaces of particles thereof with a surface layer  24  containing metallic cobalt or a cobalt oxide. The copper-coated hydrogen absorbing alloy powder A affords improved electric conductivity, and the cobalt-coated hydrogen absorbing alloy powder B gives improved ability to absorb and desorb hydrogen, whereby the electrode is improved in different battery characteristics, such as cycle characteristics, high-rate discharge characteristics and infernal pressure characteristics, at the same time.

TECHNICAL FIELD

The present invention relates to hydrogen absorbing alloy electrodes foruse as negative electrodes of alkaline secondary batteries such asnickel-metal hydride batteries and a process for fabricating theelectrode, and more particularly to improvements in the surfacecharacteristics of hydrogen absorbing alloys.

BACKGROUND ART

While nickel-cadmium batteries and lead acid batteries are already inwide use as secondary batteries, it is desired especially in recentyears to develop secondary batteries which have a high energy densityand are yet clean with advances in compact information devices such ascellular phones and notebook computers. Accordingly attention has beendirected to closed-type nickel-metal hydride batteries which are freefrom harmful substances such as cadmium and lead and wherein anelectrode of hydrogen absorbing alloy is used as the negative electrode.

Nickel-metal hydride batteries comprise a negative electrode of hydrogenabsorbing alloy, a positive electrode of nickel, an alkalineelectrolyte, a separator, etc. The hydrogen absorbing alloy electrodeserving as the negative electrode is prepared by adding a binder to ahydrogen absorbing alloy powder obtained by pulverizing a hydrogenabsorbing alloy ingot, and forming the mixture to a shape of anelectrode. The hydrogen absorbing alloys developed for use in thenegative electrode include those of the Misch metal type comprising aMisch metal (hereinafter referred to as “Mm”) which is a mixture ofrare-earth elements and Laves phase hydrogen absorbing alloys.

With alkaline secondary batteries, such as nickel-metal hydridebatteries, wherein a negative electrode of hydrogen absorbing alloy isused, a gas-phase reaction and an electrochemical reaction proceed atthe same time on the surface of the hydrogen absorbing alloy by virtueof the contact of the alloy surface with the alkaline electrolyte. Morespecifically, in the relationship between the hydrogen pressure and thetemperature, hydrogen is absorbed by the hydrogen absorbing alloy, orthe hydrogen absorbing alloy desorbs hydrogen (gas-phase reaction). Inthe relationship between the voltage and the current, on the other hand,application of voltage (charging) causes absorption of hydrogen by thehydrogen absorption alloy of the hydrogen produced by the electrolysisof water, and delivery of current (discharge) causes oxidation ofhydrogen to water (electrochemical reaction).

To promote such gas-phase reaction and electrochemical reaction on thehydrogen absorbing alloy surface, a powder of electric conductor isadded to the electrode, or the hydrogen absorbing alloy surface ischemically treated, whereas if the attempt to improve the hydrogenabsorbing alloy is made singly, various characteristics of the hydrogenabsorbing alloy are merely improved partially, and appearance of a newlimiting factor hampers great improvements in the characteristics of thealkaline secondary battery.

An object of the present invention is to provide a hydrogen absorbingalloy electrode which is given outstanding battery characteristics bysimultaneously ameliorating problems encountered with hydrogen absorbingalloys in the electric conductivity, gas affinity, etc. which areassociated with the gas-phase reaction and electrochemical reaction.

DISCLOSURE OF THE INVENTION

A hydrogen absorbing alloy electrode embodying the present inventionconsists mainly of a mixture of a first hydrogen absorbing alloy powderA formed on surfaces of particles thereof with a surface layer 22containing metallic copper or a copper oxide and a second hydrogenabsorbing alloy powder B formed on surfaces of particles thereof with asurface layer 24 containing metallic cobalt or a cobalt oxide.

Copper is an element having a extremely high electric conductivity. Thesurface layer 22 formed on the surfaces of hydrogen absorbing alloyparticles 21 and containing metallic copper or a copper oxide promotesthe migration of electrons due to the acceptance of hydrogen atoms bythe surface of the hydrogen absorbing alloy, contributing toimprovements in high-rate discharge characteristics and suppressing theformation of gas due to a side reaction. Cobalt has high affinity forgases and has catalytic activity to promote dissociation of hydrogenmolecules and combination thereof with oxygen atoms on the surface ofthe hydrogen absorbing alloy. Accordingly, the surface layer 24 formedon the surfaces of hydrogen absorbing alloy particles 23 and containingmetallic cobalt or a cobalt oxide promotes absorption of hydrogen by thehydrogen absorbing alloy and a water forming reaction due to therecombination of the gaseous oxygen produced on the surface of thepositive electrode and the hydrogen atoms produced on the surface of thenegative electrode. This consequently suppresses the rise in the batteryinternal pressure during the charge-discharge reactions and leads toimproved cycle characteristics.

Accordingly, the alkaline secondary batteries incorporating the hydrogenabsorbing alloy electrode of the invention exhibit unprecedentedexcellent battery characteristics which are attributable to the presenceof the two kinds of hydrogen absorbing alloys having surface layers oftwo kinds of metals which are different in effect.

Moreover, the surface layers of the hydrogen absorbing alloy powdersprotect the exposed active sites on the surface of the alloy fromoxidation, thereby contributing also to improvements in cyclecharacteristics.

Stated specifically, the first hydrogen absorbing alloy powder A issmaller than the second hydrogen absorbing alloy powder B in particlesize for use in the mixture. Further the amount (by weight) of the firsthydrogen absorbing alloy powder A is not greater than the amount (byweight) of the second hydrogen absorbing alloy powder B in the mixtureto be used.

The first hydrogen absorbing alloy powder A is itself capable ofabsorbing and desorbing hydrogen, therefore present as an activesubstance and serves also as a conductive material because the copperatoms present in the surface layer have satisfactory electricconductivity. Accordingly, when the particles of the first hydrogenabsorbing alloy powder A form a bridge between the particles of thesecond hydrogen absorbing alloy powder B or between the particles of thesecond hydrogen absorbing alloy powder B and a current collector,electrons can be transmitted efficiently. Further when the firsthydrogen absorbing alloy powder A is smaller than the second hydrogenabsorbing alloy powder B in particle size, the first powder A is presentin the interstices between the particles of the second powder B, givingan increased fractional solids content to the electrode alloy, so thatthe alkaline secondary battery incorporating such a hydrogen absorbingalloy electrode exhibits satisfactory battery characteristics.

To obtain satisfactory battery characteristics by using the hydrogenabsorbing alloy electrode in alkaline secondary batteries, it is desiredthat the amount of the second hydrogen absorbing alloy powder B begreater than the amount of the first hydrogen absorbing alloy powder Ain the mixture as will be apparent from the experimental result to begiven later.

A process for preparing the hydrogen absorbing alloy electrode embodyingthe present invention has a step of preparing a first hydrogen absorbingalloy powder A, a step of preparing a second hydrogen absorbing alloypowder B, and a step of preparing an electrode by mixing together thetwo kinds of hydrogen absorbing alloy powders obtained by these stepsand forming the mixture into a predetermined shape. In the step ofpreparing a first hydrogen absorbing alloy powder A, the alloy powder issubjected to a surface treatment with an acid solution containing acopper chloride (CuCl₂) and adjusted to a pH value of 0.7 to 2.0, and inthe step of preparing a second hydrogen absorbing alloy powder B, thealloy powder is subjected to a surface treatment with an acid solutioncontaining a cobalt chloride (CoCl₂) or cobalt hydroxide (Co(OH)₂) andadjusted to a pH value of 0.7 to 2.0.

A film of an oxide or the like is generally formed on the surfaces ofhydrogen absorbing alloys by spontaneous oxidation. When an electrode isfabricated from such a hydrogen absorbing alloy and used as the negativeelectrode of an alkaline secondary battery, the hydrogen absorbing alloyis initially low in activity, giving lower characteristics to thebattery initially or at low temperatures.

The process for preparing a hydrogen absorbing alloy electrode embodyingthe invention has a step of removing the oxide layer in an acid solutionto obviate the drawback. Stated specifically, the hydrogen absorbingalloy is immersed in an acid solution to dissolve the oxide layer on thesurface. Active sites are exposed on the hydrogen absorbing alloysurface by this step, and a surface layer of copper or a copper oxide,or cobalt or a cobalt oxide is thereafter formed on the hydrogenabsorbing alloy surface, whereby the active sites can be protected fromre-oxidation.

If the acid solution has too low an initial pH value in this step, atough oxide film will be formed on the hydrogen absorbing alloy surface,whereas if the initial pH value is excessively high, the oxide film willnot be completely removed, failing to give improved batterycharacteristics to the hydrogen absorbing alloy electrode. It istherefore desired that the acid solution for use in this step be 0.7 to2.0 in initial pH value.

Stated further specifically, the solution for use in the step ofpreparing the first hydrogen absorbing alloy powder A contains 1.0 wt. %to 5.0 wt. % of a copper chloride (CuCl₂) based on the weight of thehydrogen absorbing alloy. Further the solution for use in the step ofpreparing the second hydrogen absorbing alloy powder B contains 1.0 wt.% to 5.0 wt. % of a cobalt chloride (CoCl₂) based on the weight of thehydrogen absorbing alloy. In place of cobalt chloride (CoCl₂), a cobalthydroxide (Co(OH)₂) can be used in an amount of 0.3 wt. % to 1.0 wt. %based on the weight of the hydrogen absorbing alloy.

In the case where the hydrogen absorbing alloy powder protected by thesurface layer comprising copper or cobalt against oxidation as with airis used for the hydrogen absorbing alloy electrode of the alkalinesecondary battery, the outer surface coating of the surface layer partlydissolves in the electrolyte and reduces in thickness. The surface layerremaining on the hydrogen absorbing alloy surface at this time hampersthe absorption or desorption of hydrogen by the hydrogen absorbing alloyif having an excessive thickness, or permits the oxygen present in theelectrolyte to oxidize the active sites if having too small a thickness.

Accordingly, the concentration of copper chloride (CuCl₂) in the acidsolution for surface-treating the starting hydrogen absorbing alloypowder therein in the step of preparing the first hydrogen absorbingalloy powder A is preferably 1.0 wt. % to 5.0 wt. % based on the weightof the hydrogen absorbing alloy. On the other hand, the concentration ofcobalt chloride (CoCl₂) in the acid solution for surface-treating thestarting hydrogen absorbing alloy powder therein in the step ofpreparing the second hydrogen absorbing alloy powder B is preferably 1.0wt. % to 5.0 wt. % based on the weight of the hydrogen absorbing alloy.When a cobalt hydroxide (Co(OH)₂) is used, the concentration thereof ispreferably 0.3 wt. % to 1.0 wt. % based on the weight of the hydrogenabsorbing alloy.

The alkaline secondary battery incorporating the hydrogen absorbingalloy electrode of the invention exhibits a synergistic effectattributable to the presence of the two kinds of hydrogen absorbingalloy powders which are different in properties to serve as negativeelectrode active substances, i.e., a hydrogen absorbing alloy powder Ahaving a surface layer containing highly conductive copper and ahydrogen absorbing alloy powder B having a surface layer containingcobalt which is in battery internal pressure characteristics and cyclecharacteristics. The alkaline secondary battery is further superior inbattery characteristics to the one fabricated with use of only one ofthese powders for improvement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged schematic view in section showing an activematerial layer of a hydrogen absorbing alloy electrode embodying thepresent invention.

FIG. 2 is a sectional view showing a nickel-metal hydride batteryprepared from a hydrogen absorbing alloy electrode embodying the presentinvention.

BEST MODE OF CARRYING OUT THE INVENTION

With reference to FIG. 1, the hydrogen absorbing alloy electrode of thepresent invention has an active substance layer 2 which is in the formof a mixture of a first hydrogen absorbing alloy powder A having asurface layer 22 formed on the surfaces of hydrogen absorbing alloyparticles 21 and containing metallic copper or a copper oxide(hereinafter referred to as a “copper-coated hydrogen absorbing alloypowder A”) and a second hydrogen absorbing alloy powder B having asurface layer 24 formed on the surfaces of hydrogen absorbing alloyparticles 23 and containing metallic cobalt or a cobalt oxide(hereinafter referred to as a “cobalt-coated hydrogen absorbing alloypowder B”).

The copper-coated hydrogen absorbing alloy powder A smaller than thecobalt-coated hydrogen absorbing alloy powder B in particle size ispresent in the interstices between the particles of the powder B inintimate with the particles to achieve a high fractional solids content.

The mode for the embodiment of the present invention will be describedbelow in detail.

EXAMPLE 1

(Preparation of Hydrogen Absorbing Alloy and Hydrogen Absorbing AlloyPowders)

A Mm, and Ni, Co, Al and Mn each of which is a single metal having apurity of 99.9% were mixed together in a predetermined molar ratio,melted in an arc melting furnace having an argon atmosphere andthereafter spontaneously cooled to prepare a hydrogen absorbing alloyrepresented by the composition formulaMmNi_(3.1)Co_(1.0)Al_(0.3)Mn_(0.6) and having a crystal structure of theCaCu₅ type.

The ingot of the hydrogen absorbing alloy prepared by the above methodwas mechanically pulverized in an argon atmosphere to obtain a hydrogenabsorbing alloy powder (referred to as alloy P1 below) with a meanparticle size of 30 μm and a hydrogen absorbing alloy powder (referredto as alloy P2 below) with a mean particle size of 60 μm.

(The Surface Treatment of a Hydrogen Absorbing Powder)

To an aqueous hydrochloric acid solution was added a copper chloride(CuCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 1.0 to prepare a treating solution (acidsolution). The alloy P1 was immersed in the treating solution withstirring for 15 minutes, then filtered off under suction, thereafterwashed with water and dried to obtain a copper-coated hydrogen absorbingalloy powder A.

To an aqueous hydrochloric acid solution was added a cobalt chloride(CoCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 1.0 to prepare a treating solution (acidsolution). The alloy P2 was immersed in the treating solution withstirring for 15 minutes, then filtered off under suction, thereafterwashed with water and dried to obtain a cobalt-coated hydrogen absorbingalloy powder B.

(Preparation of Electrodes)

The copper-coated hydrogen absorbing alloy powder A and thecobalt-coated hydrogen absorbing alloy powder B thus surface-treatedwere mixed together in a ratio by weight of 1:9 to obtain a negativeelectrode active substance. 100 parts by weight of the active substanceand 20 parts by weight of an aqueous solution of 5 wt. % of polyethyleneoxide serving as a binder were mixed together into a paste. The pastewas applied to opposite surfaces of a core comprising a nickel-platedpunched metal sheet, dried at room temperature and thereafter cut to aspecified size to prepare a negative electrode.

A known sintered nickel electrode was used as a positive electrode.

(Fabrication of Nickel-metal Hydride Battery)

A nickel-metal hydride battery [Sample No. 1] of size AA and thepositive electrode predominant type was fabricated using the abovepositive electrode and negative electrode, a separator ofalkali-resistant non-woven fabric and an electrolyte comprising 30 wt. %of potassium hydroxide.

FIG. 2 shows a nickel-metal hydride battery 1 wherein a hydrogenabsorbing alloy electrode of the invention is used and which comprises apositive electrode 11, negative electrode 12, separator 13, positiveelectrode lead 14, negative electrode lead 15, positive electrodeexternal terminal 16, negative electrode can 17, closure 18, etc. Thepositive electrode 11 and the negative electrode 12 are spirally woundup into a roll with the separator 13 interposed therebetween, andencased in the can 17. The positive electrode 11 is connected to theclosure 18 by the positive electrode lead 14, and the negative electrode12 to the can 17 by the negative electrode lead 15. An insulatingpacking 20 is provided at the joint between the negative electrode can17 and the closure 18 to seal off the nickel-metal hydride battery 1. Acoil spring 19 is provided between the external terminal 16 and theclosure 18 so that when the battery internal pressure builds upabnormally, the spring is compressed to release a gas from inside thebattery to the atmosphere.

EXAMPLE 2

To an aqueous hydrochloric acid solution was added a copper chloride(CuCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 1.0 to prepare a treating solution. The alloyP2 was immersed in the treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a copper-coated hydrogen absorbing alloy powder A.

To an aqueous hydrochloric acid solution was added a cobalt chloride(CoCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 1.0 to prepare a treating solution. The alloyP1 was immersed in the treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a cobalt-coated hydrogen absorbing alloy powder B.

A nickel-metal hydride battery [Sample No. 2] according to the inventionwas fabricated in the same manner as in Example 1 with the exception ofusing a negative electrode active substance prepared by mixing togetherthe copper-coated hydrogen absorbing alloy powder A and thecobalt-coated hydrogen absorbing alloy powder B in a ratio of 1:9 byweight.

EXAMPLE 1 and 3-7

In the same manner as in Example 1, a copper-coated hydrogen absorbingalloy powder A and a cobalt-coated hydrogen absorbing alloy powder Bwere prepared.

Six kinds of nickel-metal hydride batteries [Sample Nos. 1′ and 3-7]according to the invention were fabricated in the same manner as inExample 1 with the exception of using a negative electrode activesubstance prepared by mixing together the copper-coated hydrogenabsorbing alloy powder A and the cobalt-coated hydrogen absorbing alloypowder B in a ratio of 1:19, 2:1, 3:2, 1:1, 2:3 or 1:4 by weight.

EXAMPLES 8-13

To an aqueous hydrochloric acid solution was added a copper chloride(CuCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 0.5 to prepare a treating solution. The alloyP1 was immersed in the treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a copper-coated hydrogen absorbing alloy powder A.

To an aqueous hydrochloric acid solution was added a cobalt chloride(CoCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 0.5, 0.7, 1.0, 1.5, 2.0 or 2.5 to prepare sixkinds of treating solutions. The alloy P2 was immersed in the six kindsof treating solutions with stirring for 15 minutes, then filtered offunder suction, thereafter washed with water and dried to obtain sixkinds of cobalt-coated hydrogen absorbing alloy powders B.

Six kinds of nickel-metal hydride batteries [Sample Nos. 8-13] accordingto the invention were fabricated in the same manner as in Example 1 withthe exception of using a negative electrode active substance prepared bymixing together the copper-coated hydrogen absorbing alloy powder A andeach of the cobalt-coated hydrogen absorbing alloy powders B in a ratioby weight of 1:9.

EXAMPLES 14-19

To an aqueous hydrochloric acid solution was added a copper chloride(CuCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 0.7 to prepare a treating solution. The alloyP1 was immersed in the treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a copper-coated hydrogen absorbing alloy powder A.

To an aqueous hydrochloric acid solution was added a cobalt chloride(CoCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 0.5, 0.7, 1.0, 1.5, 2.0 or 2.5 to prepare sixkinds of treating solutions. The alloy P2 was immersed in these sixkinds of treating solutions with stirring for 15 minutes, then filteredoff under suction, thereafter washed with water and dried to obtain sixkinds of cobalt-coated hydrogen absorbing alloy powders B.

Six kinds of nickel-metal hydride batteries [Sample Nos. 14-19]according to the invention were fabricated in the same manner as inExample 1 with the exception of using a negative electrode activesubstance prepared by mixing together the copper-coated hydrogenabsorbing alloy powder A and each of the cobalt-coated hydrogenabsorbing alloy powders B in a ratio of 1:9 by weight.

EXAMPLES 20-24

To an aqueous hydrochloric acid solution was added a copper chloride(CuCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 1.0 to prepare a treating solution. The alloyP1 was immersed in the treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a copper-coated hydrogen absorbing alloy powder.

To an aqueous hydrochloric acid solution was added a cobalt chloride(CoCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 0.5, 0.7, 1.5, 2.0 or 2.5 to prepare fivekinds of treating solutions. The alloy P2 was immersed in these fivekinds of treating solutions with stirring for 15 minutes, then filteredoff under suction, thereafter washed with water and dried to obtain fivekinds of cobalt-coated hydrogen absorbing alloy powders B.

Five kinds of nickel-metal hydride batteries [Sample Nos. 20-24]according to the invention were fabricated in the same manner as inExample 1 with the exception of using a negative electrode activesubstance prepared by mixing together the copper-coated hydrogenabsorbing alloy powder A and each of the cobalt-coated hydrogenabsorbing alloy powders B in a ratio of 1:9 by weight.

EXAMPLES 25-30

To an aqueous hydrochloric acid solution was added a copper chloride(CuCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 1.5 to prepare a treating solution. The alloyP1 was immersed in the treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a copper-coated hydrogen absorbing alloy powder A.

To an aqueous hydrochloric acid solution was added a cobalt chloride(CoCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 0.5, 0.7, 1.0, 1.5, 2.0 or 2.5 to prepare sixkinds of treating solutions. The alloy P2 was immersed in these sixkinds of treating solutions with stirring for 15 minutes, then filteredoff under suction, thereafter washed with water and dried to obtain sixkinds of cobalt-coated hydrogen absorbing alloy powders B.

Six kinds of nickel-metal hydride batteries [Sample Nos. 25-30]according to the invention were fabricated in the same manner as inExample 1 with the exception of using a negative electrode activesubstance prepared by mixing together the copper-coated hydrogenabsorbing alloy powder A and each of the cobalt-coated hydrogenabsorbing alloy powders B in a ratio by weight of 1:9.

EXAMPLES 31-36

To an aqueous hydrochloric acid solution was added a copper chloride(CuCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 2.0 to prepare a treating solution. The alloyP1 was immersed in the treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a copper-coated hydrogen absorbing alloy powder A.

To an aqueous hydrochloric acid solution was added a cobalt chloride(CoCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 0.5, 0.7, 1.0, 1.5, 2.0 or 2.5 to prepare sixkinds of treating solutions. The alloy P2 was immersed in these sixkinds of treating solutions with stirring for 15 minutes, then filteredoff under suction, thereafter washed with water and dried to obtain sixkinds of cobalt-coated hydrogen absorbing alloy powders B.

Six kinds of nickel-metal hydride batteries [Sample Nos. 31-36]according to the invention were fabricated in the same manner as inExample 1 with the exception of using a negative electrode activesubstance prepared by mixing together the copper-coated hydrogenabsorbing alloy powder A and each of the cobalt-coated hydrogenabsorbing alloy powders B in a ratio of 1:9 by weight.

EXAMPLES 37-42

To an aqueous hydrochloric acid solution was added a copper chloride(CuCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 2.5 to prepare a treating solution. The alloyP1 was immersed in the treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a copper-coated hydrogen absorbing alloy powder A.

To an aqueous hydrochloric acid solution was added a cobalt chloride(CoCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 0.5, 0.7, 1.0, 1.5, 2.0 or 2.5 to prepare sixkinds of treating solutions. The alloy P2 was immersed in these sixkinds of treating solutions with stirring for 15 minutes, then filteredoff under suction, thereafter washed with water and dried to obtain sixkinds of cobalt-coated hydrogen absorbing alloy powders B.

Six kinds of nickel-metal hydride batteries [Sample Nos. 37-42]according to the invention were fabricated in the same manner as inExample 1 with the exception of using a negative electrode activesubstance prepared by mixing together the copper-coated hydrogenabsorbing alloy powder A and each of the cobalt-coated hydrogenabsorbing alloy powders B in a ratio of 1:9 by weight.

EXAMPLES 43-47

To an aqueous hydrochloric acid solution was added a copper chloride(CuCl₂) in an amount of 0.5 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 1.0 to prepare a treating solution. The alloyP1 was immersed in the treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a copper-coated hydrogen absorbing alloy powder A.

To an aqueous hydrochloric acid solution was added a cobalt chloride(CoCl₂) in an amount of 0.5, 1.0, 3.0, 5.0 or 7.0 wt. % based on thehydrogen absorbing alloy to be immersed in the solution later, and theresulting solutions were adjusted to a pH value of 1.0 to prepare fivekinds of treating solutions. The alloy P2 was immersed in these treatingsolutions with stirring for 15 minutes, then filtered off under suction,thereafter washed with water and dried to obtain five kinds ofcobalt-coated hydrogen absorbing alloy powders B.

Five kinds of nickel-metal hydride batteries [Sample Nos. 43-47]according to the invention were fabricated in the same manner as inExample 1 with the exception of using a negative electrode activesubstance prepared by mixing together the copper-coated hydrogenabsorbing alloy powder A and each of the cobalt-coated hydrogenabsorbing alloy powders B in a ratio of 1:9 by weight.

EXAMPLES 48-52

To an aqueous hydrochloric acid solution was added a copper chloride(CuCl₂) in an amount of 1.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 1.0 to prepare a treating solution. The alloyP1 was immersed in the treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a copper-coated hydrogen absorbing alloy powder A.

To an aqueous hydrochloric acid solution was added a cobalt chloride(CoCl₂) in an amount of 0.5, 1.0, 3.0, 5.0 or 7.0 wt. % based on thehydrogen absorbing alloy to be immersed in the solution later, and theresulting solutions were adjusted to a pH value of 1.0 to prepare fivekinds of treating solutions. The alloy P2 was immersed in these treatingsolutions with stirring for 15 minutes, then filtered off under suction,thereafter washed with water and dried to obtain five kinds ofcobalt-coated hydrogen absorbing alloy powders B.

Five kinds of nickel-metal hydride batteries [Sample Nos. 48-52]according to the invention were fabricated in the same manner as inExample 1 with the exception of using a negative electrode activesubstance prepared by mixing together the copper-coated hydrogenabsorbing alloy powder A and each of the cobalt-coated hydrogenabsorbing alloy powders B in a ratio of 1:9 by weight.

EXAMPLES 53-56

To an aqueous hydrochloric acid solution was added a copper chloride(CuCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 1.0 to prepare a treating solution. The alloyP1 was immersed in the treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a copper-coated hydrogen absorbing alloy powder A.

To an aqueous hydrochloric acid solution was added a cobalt chloride(CoCl₂) in an amount of 0.5, 1.0, 5.0 or 7.0 wt. % based on the hydrogenabsorbing alloy to be immersed in the solution later, and the resultingsolutions were adjusted to a pH value of 1.0 to prepare four kinds oftreating solutions. The alloy P2 was immersed in these treatingsolutions with stirring for 15 minutes, then filtered off under suction,thereafter washed with water and dried to obtain four kinds ofcobalt-coated hydrogen absorbing alloy powders B.

Four kinds of nickel-metal hydride batteries [Sample Nos. 53-56]according to the invention were fabricated in the same manner as inExample 1 with the exception of using a negative electrode activesubstance prepared by mixing together the copper-coated hydrogenabsorbing alloy powder A and each of the cobalt-coated hydrogenabsorbing alloy powders B in a ratio of 1:9 by weight.

EXAMPLES 57-61

To an aqueous hydrochloric acid solution was added a copper chloride(CuCl₂) in an amount of 5.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 1.0 to prepare a treating solution. The alloyP1 was immersed in the treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a copper-coated hydrogen absorbing alloy powder A.

To an aqueous hydrochloric acid solution was added a cobalt chloride(CoCl₂) in an amount of 0.5, 1.0, 3.0, 5.0 or 7.0 wt. % based on thehydrogen absorbing alloy to be immersed in the solution later, and theresulting solutions were adjusted to a pH value of 1.0 to prepare fivekinds of treating solutions. The alloy P2 was immersed in these treatingsolutions with stirring for 15 minutes, then filtered off under suction,thereafter washed with water and dried to obtain six kinds ofcobalt-coated hydrogen absorbing alloy powders B.

Five kinds of nickel-metal hydride batteries [Sample Nos. 57-61]according to the invention were fabricated in the same manner as inExample 1 with the exception of using a negative electrode activesubstance prepared by mixing together the copper-coated hydrogenabsorbing alloy powder A and each of the cobalt-coated hydrogenabsorbing alloy powders B in a ratio of 1:9 by weight.

EXAMPLES 62-66

To an aqueous hydrochloric acid solution was added a copper chloride(CuCl₂) in an amount of 7.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 1.0 to prepare a treating solution. The alloyP1 was immersed in the treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a copper-coated hydrogen absorbing alloy powder A

To an aqueous hydrochloric acid solution was added a cobalt chloride(CoCl₂) in an amount of 0.5, 1.0, 3.0, 5.0 or 7.0 wt. % based on thehydrogen absorbing alloy to be immersed in the solution later, and theresulting solutions were adjusted to a pH value of 1.0 to prepare fivekinds of treating solutions. The alloy P2 was immersed in these treatingsolutions with stirring for 15 minutes, then filtered off under suction,thereafter washed with water and dried to obtain five kinds ofcobalt-coated hydrogen absorbing alloy powders B.

Five kinds of nickel-metal hydride batteries [Sample Nos. 62-66]according to the invention were fabricated in the same manner as inExample 1 with the exception of using a negative electrode activesubstance prepared by mixing together the copper-coated hydrogenabsorbing alloy powder A and each of the cobalt-coated hydrogenabsorbing alloy powders B in a ratio of 1:9 by weight.

EXAMPLES 67-71

To an aqueous hydrochloric acid solution was added a copper chloride(CuCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 1.0 to prepare a treating solution. The alloyP1 was immersed in the treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a copper-coated hydrogen absorbing alloy powder A.

To an aqueous hydrochloric acid solution was added a cobalt hydroxide(Co(OH)₂) in an amount of 0.1, 0.3, 0.5, 1.0 or 1.5 wt. % based on thehydrogen absorbing alloy to be immersed in the solution later, and theresulting solution was adjusted to a pH value of 1.0 to prepare fivekinds of treating solutions. The alloy P2 was immersed in these treatingsolutions with stirring for 15 minutes, then filtered off under suction,thereafter washed with water and dried to obtain five kinds ofcobalt-coated hydrogen absorbing alloy powders B.

Five kinds of nickel-metal hydride batteries [Sample Nos. 67-71]according to the invention were fabricated in the same manner as inExample 1 with the exception of using a negative electrode activesubstance prepared by mixing together the copper-coated hydrogenabsorbing alloy powder A and each of the cobalt-coated hydrogenabsorbing alloy powders B in a ratio of 1:9 by weight.

EXAMPLES 72-142

A hydrogen absorbing alloy was prepared in the same manner as in Example1 and made into a hydrogen absorbing alloy powder up to 30 μm inparticle size (hereinafter referred to as the “alloy P3”) and a hydrogenabsorbing alloy powder 30 μm to 60 μm in particle size (hereinafterreferred to as the “alloy P4”) by the gas atomization process.

Seventy-one kinds of nickel-metal hydride batteries [Sample Nos. 72-142]according to the invention and corresponding to those of Examples 1 to71, respectively, were fabricated in the same manner as in theseexamples with the exception of using the alloy P3 in place of the alloyP1 and the alloy P4 in place of the alloy P2.

Comparative Example 1

An aqueous hydrochloric acid solution containing no copper chloride(CuCl₂) was adjusted to a pH value of 1.0 to prepare a treatingsolution, in which the alloy P1 was immersed with stirring for 15minutes, then filtered off under suction, thereafter washed with waterand dried to obtain a hydrogen absorbing alloy powder. A comparativebattery [Sample No. C1] was fabricated in the same manner as in Example1 with the exception of using a negative electrode active substanceprepared by mixing together this hydrogen absorbing alloy powder and anickel powder serving as a conductive agent in a ratio of 9:1 by weight.

Comparative Example 2

To an aqueous hydrochloric acid solution was added a cobalt chloride(CoCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 1.0 to prepare a treating solution. The alloyP2 was immersed in this treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a cobalt-coated hydrogen absorbing alloy powder B. Acomparative battery [Sample No. C2] was fabricated in the same manner asin Example 1 with the exception of using a negative electrode activesubstance prepared by mixing together this cobalt-coated hydrogenabsorbing alloy powder B and a nickel powder serving as a conductiveagent in a ratio of 9:1 by weight.

Comparative Example 3

To an aqueous hydrochloric acid solution was added a copper chloride(CuCl₂) in an amount of 3.0 wt. % based on the hydrogen absorbing alloyto be immersed in the solution later, and the resulting solution wasadjusted to a pH value of 1.0 to prepare a treating solution. The alloyP1 was immersed in this treating solution with stirring for 15 minutes,then filtered off under suction, thereafter washed with water and driedto obtain a copper-coated hydrogen absorbing alloy powder A. Acomparative battery [Sample No. C3] was fabricated in the same manner asin Example 1 with the exception of using a negative electrode activesubstance prepared by mixing together this copper-coated hydrogenabsorbing alloy powder A and a nickel powder serving as a conductiveagent in a ratio of 9:1 by weight.

Comparative Examples 4-6

Three kinds of comparative batteries [Sample Nos. C4-C6] were fabricatedin the same manner as in Comparative Examples 1 to 3 with the exceptionof using the alloy P3 in place of the alloy P1 and the alloy P4 insteadof the alloy P2.

Charge-Discharge Cycle Test

The cycle of charging the battery at a current value of 1.5 C for 0.8hour and thereafter discharging the battery at a current value of 1.5 Cto 1.0 V was repeated at room temperature. The battery was checked fordischarge capacity (mAh) in the 500th cycle.

Battery Internal Pressure Characteristic

The battery was charged at a current value of 1.0 C at room temperatureand checked for internal pressure to measure the charging time taken forthe internal pressure to reach 10 kgf/cm².

High-Rate Discharge Characteristics

The battery was charged at 0.2 C for 6 hours and thereafter dischargedat a current value of 6.0 C to 1.0 V to determine the discharge capacity(mAh).

Table 1 shows the experimental result achieved with use of thecopper-coated hydrogen absorbing alloy powder A and the cobalt-coatedhydrogen absorbing alloy powder B which were prepared from amechanically pulverized hydrogen absorbing alloy to substantiate theeffect of the particles sizes of the powders.

TABLE 1 ALLOY A/ HIGH - ALLOY B CAPACITY INTERNAL RATE (CON- AFTERPRESSURE DISCHARGE DUCTIVE 500 CHARACTER- CHARACTER- SAMPLE AGENT/CYCLES ISTICS ISTICS NO. ALLOY) mAh min mAh 1 1/9 860 145 820 2 1/9 840135 810 C1 (1/9) 720 110 790 C2 (1/9) 800 135 800 C3 (1/9) 760 115 805

With reference to Table 1, the nickel-metal hydride batteries whereinthe hydrogen absorbing alloy electrode of the invention is used arefound to be superior with respect to all of the cycle characteristics,internal pressure characteristics and high-rate dischargecharacteristics when the comparative battery [Sample No. C1] wherein theconventional hydrogen absorbing alloy having no metallic coating is usedand the comparative batteries [Sample Nos. C2 and C3] wherein one of thecopper-coated hydrogen absorbing alloy powder A and the cobalt-coatedhydrogen absorbing alloy powder B is used are compared with thenickel-metal hydride batteries [samples Nos. 1 and 2] wherein thesepowders A and B are used for the hydrogen absorbing alloy electrodeaccording to the invention.

Further a comparison between the nickel-metal hydride batteries [SampleNos. 1 and 2] wherein the hydrogen absorbing alloy electrode of theinvention is used reveals that the cobalt-coated hydrogen absorbingalloy powder B is preferably greater than the copper-coated hydrogenabsorbing alloy powder A in particle size when to be used in combinationtherewith for hydrogen absorbing alloy electrodes of the invention.

Table 2 shows the experimental result achieved as to the weight ratiobetween the copper-coated hydrogen absorbing alloy powder A and thecobalt-coated hydrogen absorbing alloy powder B to be mixed together inpreparing hydrogen absorbing alloy electrodes of the invention.

TABLE 2 ALLOY A/ HIGH - ALLOY B CAPACITY INTERNAL RATE (CON- AFTERPRESSURE DISCHARGE DUCTIVE 500 CHARACTER- CHARACTER- SAMPLE AGENT/CYCLES ISTICS ISTICS NO. ALLOY) mAh min mAh 1 1/9 850 145 820 1′  1/19860 145 820 3 2/1 810 135 800 4 3/2 810 135 800 5 1/1 840 135 815 6 2/3845 135 820 7 1/4 850 140 825 C1 (1/9) 720 11o 790 C2 (1/9) 800 135 800C3 (1/9) 760 115 805

Table 2 reveals that as compared with the comparative batteries [SampleNos. C1-C3], the nickel-metal hydride batteries [Sample Nos. 1, 1′, 5-7]of the invention wherein the cobalt-coated hydrogen absorbing alloypowder B is equivalent to or greater than the copper-coated hydrogenabsorbing alloy powder A in content by weight are greatly improved inall of the discharge capacity after 500 cycles, internal pressurecharacteristics and high-rate discharge characteristics. It isespecially desirable that the copper-coated hydrogen absorbing alloypowder A be at least 5 wt. % to not greater than 50 wt. %. Further acomparison of the batteries of the invention with the comparativebatteries [Sample Nos. C2 and C3] suggests that the advantage of theinvention is attributable largely to the synergistic effect due to thepresence of both the copper-coated hydrogen absorbing alloy powder andthe cobalt-coated hydrogen absorbing alloy powder.

Table 3 shows the experimental result achieved with respect to the pH ofthe treating solutions used in the oxide film removing steps forpreparing the copper-coated hydrogen absorbing alloy powder A and thecobalt-coated hydrogen absorbing alloy powder B for hydrogen absorbingalloy electrodes of the invention.

TABLE 3 HIGH - ALLOY ALLOY CAPAC- RATE A B ITY INTERNAL DIS- TREAT-TREAT- AFTER PRESSURE CHARGE SAM- ING ING 500 CHARAC- CHARAC- PLE SOLN.SOLN. CYCLES TERSTICS TERSTICS NO. pH pH mAh min mAh  8 0.5 0.5 800 135805  9 0.5 0.7 810 140 805 10 0.5 1.0 810 140 805 11 0.5 1.5 810 140 80512 0.5 2.0 810 140 805 13 0.5 2.5 800 135 805 14 0.7 0.5 815 135 815 150.7 0.7 855 145 820 16 0.7 1.0 860 150 825 17 0.7 1.5 860 145 820 18 0.72.0 850 145 820 19 0.7 2.5 815 135 815 20 1.0 0.5 815 135 810 21 1.0 0.7860 145 820  1 1.0 1.0 860 145 825 22 1.0 1.5 860 145 825 23 1.0 2.0 855145 820 24 1.0 2.5 815 135 815 25 1.5 0.5 820 135 810 26 1.5 0.7 855 145820 27 1.5 1.0 860 145 825 28 1.5 1.5 855 145 820 29 1.5 2.0 855 145 82030 1.5 2.5 820 135 810 31 2.0 0.5 815 135 810 32 2.0 0.7 850 140 820 332.0 1.0 860 145 820 34 2.0 1.5 845 140 820 35 2.0 2.0 845 140 810 36 2.02.5 810 135 800 37 2.5 0.5 810 135 800 38 2.5 0.7 810 135 810 39 2.5 1.0810 135 810 40 2.5 1.5 815 135 810 41 2.5 2.0 815 135 810 42 2.5 2.5 810135 800

Table 3 reveals that the nickel-metal hydride batteries [Sample Nos. 1,15-18, 21-23, 26-29 and 32-35] of the invention are greatly improved inbattery characteristics. This indicates that the pH of the treatingsolutions suitable for removing the oxide film is in the range of 0.7 to2.0.

Tables 4 and 5 show the experimental result obtained with respect to theamount of metallic salts added to the treating solutions.

TABLE 4 HIGH - CAPAC- RATE ALLOY ALLOY ITY INTERNAL DIS- A B AFTERPRESSURE CHARGE SAM- AMOUNT AMOUNT 500 CHARAC- CHARAC- PLE OF OF CYCLESTERSTICS TERSTICS NO. CuCl2 CoCl2 mAh min mAh 43 0.5 0.5 810 135 800 440.5 1.0 815 140 800 45 0.5 3.0 815 140 800 46 0.5 5.0 815 140 800 47 0.57.0 810 135 800 48 1.0 0.5 815 135 805 49 1.0 1.0 860 145 820 50 1.0 3.0865 145 825 51 1.0 5.0 860 145 820 52 1.0 7.0 810 135 810 53 3.0 0.5 815135 810 54 3.0 1.0 855 145 825  1 3.0 3.0 860 145 820 55 3.0 5.0 860 145820 56 3.0 7.0 810 135 810 57 5.0 0.5 810 135 810 58 5.0 1.0 850 145 82059 5.0 3.0 855 145 825 60 5.0 5.0 860 145 820 61 5.0 7.0 810 135 810 627.0 0.5 815 135 805 63 7.0 1.0 815 135 805 64 7.0 3.0 815 135 805 65 7.05.0 815 135 805 66 7.0 7.0 810 135 800

TABLE 5 HIGH - CAPAC- RATE ALLOY ALLOY ITY INTERNAL DIS- A B AFTERPRESSURE CHARGE SAM- AMOUNT AMOUNT 500 CHARAC- CHARAC- PLE OF OF CYCLESTERSTICS TERSTICS NO. CuCl2 Co(OH)2 mAh min mAh 67 3.0 0.1 810 135 81068 3.0 0.3 855 145 825 69 3.0 0.5 860 145 825 70 3.0 1.0 850 145 820 713.0 1.5 810 135 810

As shown in Table 4, the nickel-metal hydride batteries [Sample Nos. 1,49-51, 54, 55 and 58-60] of the invention are greatly improved inbattery characteristics which batteries were obtained by using an acidsolution containing a copper chloride (CuCl₂) amount of 1.0 to 5.0 wt. %based on the weight of the hydrogen absorbing alloy to be treated and anacid solution containing a cobalt chloride (COCl₂) in an amount of 1.0to 5.0 wt. % based on the weight of the hydrogen absorbing alloy to betreated in forming a metallic coating on each hydrogen absorbing alloy,and using the resulting hydrogen absorbing alloy powders as activesubstances. To give an optimum thickness to the respective metalliccoatings of the copper-coated hydrogen absorbing alloy powder A andcobalt-coated hydrogen absorbing alloy powder B, it is thereforesuitable that the acid solutions contain 1.0 to 5.0 wt. % of copperchloride (CuCl₂) and 1.0 to 5.0 wt. % of cobalt chloride (CoCl₂),respectively, based on the weight of the hydrogen absorbing alloy.

Table 5 reveals that satisfactory battery characteristics are exhibitedalso by the nickel-metal hydride batteries [Sample Nos. 68-70] of theinvention which were obtained by using a cobalt hydroxide (CO(OH)₂) inplace of cobalt chloride (COCl₂) in preparing the cobalt-coated hydrogenabsorbing alloy. Accordingly, the optimum amount of cobalt hydroxide(CO(OH)₂) to be incorporated into the acid solution is 0.3 to 1.0 wt. %based on the weight of the hydrogen absorbing alloy.

Further Tables 6 to 10 show the experimental result obtained by thenickel-metal hydride batteries fabricated in the same manner as abovewith the exception of using hydrogen absorbing alloy powders prepared bythe gas atomization process as starting materials.

TABLE 6 ALLOY A/ HIGH - ALLOY B CAPACITY INTERNAL RATE (CON- AFTERPRESSURE DISCHARGE DUCTIVE 500 CHARACTER- CHARACTER- SAMPLE AGENT/CYCLES ISTICS ISTICS NO. ALLOY) mAh min mAh 72 1/9 860 145 825 73 1/9845 140 815 C4 (1/9) 725 105 800 C5 (1/9) 795 125 800 C6 (1/9) 755 120805

TABLE 6 ALLOY A/ HIGH - ALLOY B CAPACITY INTERNAL RATE (CON- AFTERPRESSURE DISCHARGE DUCTIVE 500 CHARACTER- CHARACTER- SAMPLE AGENT/CYCLES ISTICS ISTICS NO. ALLOY) mAh min mAh 72 1/9 860 145 825 73 1/9845 140 815 C4 (1/9) 725 105 800 C5 (1/9) 795 125 800 C6 (1/9) 755 120805

TABLE 8 HIGH - ALLOY ALLOY CAPAC- RATE A B ITY INTERNAL DIS- TREAT-TREAT- AFTER PRESSURE CHARGE SAM- ING ING 500 CHARAC- CHARAC- PLE SOLN.SOLN. CYCLES TERSTICS TERSTICS NO. pH pH mAh min mAh 79 0.5 0.5 800 130805 80 0.5 0.7 805 140 805 81 0.5 1.0 815 140 810 82 0.5 1.5 810 140 80583 0.5 2.0 810 135 805 84 0.5 2.5 800 135 805 85 0.7 0.5 810 135 810 860.7 0.7 855 145 820 87 0.7 1.0 855 150 825 88 0.7 1.5 855 145 825 89 0.72.0 850 145 825 90 0.7 2.5 815 130 815 91 1.0 0.5 820 130 810 92 1.0 0.7860 145 820 72 1.0 1.0 860 145 825 93 1.0 1.5 855 145 820 94 1.0 2.0 855140 820 95 1.0 2.5 815 130 815 96 1.5 0.5 820 130 815 97 1.5 0.7 855 145820 98 1.5 1.0 855 150 825 99 1.5 1.5 855 150 820 100  1.5 2.0 850 145820 101  1.5 2.5 815 135 815 102  2.0 0.5 815 135 815 103  2.0 0.7 850140 820 104  2.0 1.0 855 145 825 105  2.0 1.5 845 140 820 106  2.0 2.0845 140 820 107  2.0 2.5 810 135 800 108  2.5 0.5 810 130 805 109  2.50.7 810 135 805 110  2.5 1.0 810 130 810 111  2.5 1.5 815 130 805 112 2.5 2.0 810 130 805 113  2.5 2.5 810 130 800

TABLE 9 HIGH - CAPAC- RATE ALLOY ALLOY ITY INTERNAL DIS- A B AFTERPRESSURE CHARGE SAM- AMOUNT AMOUNT 500 CHARAC- CHARAC- PLE OF OF CYCLESTERSTICS TERSTICS NO. CuCl2 CoCl2 mAh min mAh 114 0.5 0.5 810 130 800115 0.5 1.0 815 135 805 116 0.5 3.0 815 140 805 117 0.5 5.0 815 135 805118 0.5 7.0 810 130 805 119 1.0 0.5 810 135 805 120 1.0 1.0 860 140 825121 1.0 3.0 865 145 825 122 1.0 5.0 860 145 825 123 1.0 7.0 815 130 810124 3.0 0.5 815 130 810 125 3.0 1.0 860 145 825  72 3.0 3.0 860 145 825126 3.0 5.0 860 145 820 127 3.0 7.0 815 130 805 128 5.0 0.5 810 130 805129 5.0 1.0 850 145 825 130 5.0 3.0 855 150 825 131 5.0 5.0 855 145 820132 5.0 7.0 815 130 810 133 7.0 0.5 810 130 810 134 7.0 1.0 815 130 810135 7.0 3.0 810 130 810 136 7.0 5.0 810 135 810 137 7.0 7.0 805 130 805

TABLE 10 HIGH - CAPAC- RATE ALLOY ALLOY ITY INTERNAL DIS- A B AFTERPRESSURE CHARGE SAM- AMOUNT AMOUNT 500 CHARAC- CHARAC- PLE OF OF CYCLESTERSTICS TERSTICS NO. CuCl2 Co(OH)2 mAh min mAh 138 3.0 0.1 815 135 815139 3.0 0.3 850 150 825 140 3.0 0.5 860 150 825 141 3.0 1.0 850 145 825142 3.0 1.5 815 130 810

Tables 6 to 10 reveal that in contract with the comparative batteries[Sample Nos. C4-C6], the nickel-metal hydride batteries [Sample Nos.72-142] of the invention are comparable to the foregoing nickel-metalhydride batteries [Sample Nos. 1-71 ] of the invention in advantages.This substantiates that the synergistic effect due to the presence oftwo kinds of hydrogen absorbing alloys which are different inproperties, i.e., the copper-coated hydrogen absorbing alloy powder Ahaving high electric conductivity and the cobalt-coated hydrogenabsorbing alloy powder B outstanding in battery internal pressurecharacteristics and cycle characteristics, is similarly availableregardless of the different methods of preparing the powders.

The present invention is not limited to the foregoing embodiments in thestructure but can be modified variously within the technical scope setforth in the appended claims. For example, while a hydrogen absorbingalloy having a crystal structure of the CaCu₅ type is used as thehydrogen absorbing alloy serving as the starting material, hydrogenabsorbing alloy electrodes can be prepared according to the inventionusing hydrogen absorbing alloys having a Laves phase structure of theC14 or C15 type.

What is claimed is:
 1. A hydrogen absorbing alloy electrode consistingmainly of a mixture of a first hydrogen absorbing alloy powder A formedon surfaces of particles thereof with a surface layer containingmetallic copper or a copper oxide and a second hydrogen absorbing alloypowder B formed on surfaces of particles thereof with a surface layercontaining metallic cobalt or a cobalt oxide.
 2. A hydrogen absorbingalloy electrode according to claim 1 wherein the first hydrogenabsorbing alloy powder A is smaller than the second hydrogen absorbingalloy powder B in particle size.
 3. A hydrogen absorbing alloy electrodeaccording to claim 1 wherein the amount of the first hydrogen absorbingalloy powder A is not greater than the amount of the second hydrogenabsorbing alloy powder B in the mixture.
 4. A hydrogen absorbing alloyelectrode according to claim 1 wherein the proportion of the secondhydrogen absorbing alloy powder B in the mixture is 5 wt. % to 50 wt. %.5. A process for producing a hydrogen absorbing alloy electrode having astep of preparing a first hydrogen absorbing alloy powder A, a step ofpreparing a second hydrogen absorbing alloy powder B, and a step ofpreparing an electrode by mixing together the two kinds of hydrogenabsorbing alloy powders obtained by these steps and forming the mixtureinto a predetermined shape, the first hydrogen absorbing alloy powder Abeing prepared by surface-treating a hydrogen absorbing alloy powderwith an acid solution containing a copper chloride and adjusted to a pHvalue of 0.7 to 2.0, the second hydrogen absorbing alloy powder B beingprepared by surface-treating a hydrogen absorbing alloy powder with anacid solution containing a cobalt chloride or cobalt hydroxide andadjusted to a pH value of 0.7 to 2.0.
 6. A process for producing ahydrogen absorbing alloy electrode according to claim 5 wherein thecopper chloride is contained in an amount of 1.0 wt. % to 5.0 wt. %based on the weight of the hydrogen absorbing alloy, and the cobaltchloride is contained in an amount of 1.0 wt. % to 5.0 wt. % based onthe weight of the hydrogen absorbing alloy.
 7. A process for producing ahydrogen absorbing alloy electrode according to claim 5 wherein thecobalt hydroxide is contained in an amount of 0.3 wt. % to 1.0 wt. %based on the weight of the hydrogen absorbing alloy.